With continuously decreasing semiconductor device dimensions and increasing device-packaging densities, the packaging of semiconductor devices has continued to gain in importance. In the electronics industry, the continuing goal has been to reduce the size of electronic devices such as in digital cameras and camcorders.
In the past, integrated circuits were packaged in leadframe packages using wire bonds using metal wires, but the packaging technology has been moving towards ball bond packages using solder balls, which allow for a higher density of connections.
Because the marketplace is in the process of transition, it is desirable to mix both wire and ball bonds in the same integrated circuit package.
Unfortunately, the requirements for wire and ball bonds are different. For example, aluminum (Al) is typically used for the wires because it is a very easily bent with little springback. Al bonds well to pads of Al. On the other hand, tin (Sn) solder is typically used for the balls because it has a high surface tension for forming small balls. Sn solder bonds well to pads of copper (Cu), which is a very good soldering wettable material.
The above results in the requirement for Al bond pads for wire bonds and Cu bond pads for ball bonds. Unfortunately, Al wire does not bond well with Cu bond pads and Sn solder does not bond well with Al bond pads.
In the past, Al was used in the final metal pads during integrated circuit fabrication, and Cu pads were made by thin-film deposition over the Al pads by an additional step of evaporation or sputtering. The thin-film layers of Cu are referred to as “under bump metallurgy” (UBM). The UBM plays a critical role as an adhesion layer between Al pads and solder bumps. To get different metal pads in one integrated circuit package, various approaches have been attempted, but have had the problems of requiring different masks to protect the Al pad in order to obtain a desired UBM.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.